Sallust (c. 86-34 B.C.), Roman historian, who modeled his work on Thucydides.

The editors' note identifies the wrong Sallust. Of course a man of FitzGerald's education and reading would have heard of the Roman historian Sallust. But another Sallust is meant here, not the well-known historian of the 1st century B.C. but the lesser-known philosopher of the 4th century A.D., usually called Sallustius. I don't have access to Arthur Darby Nock's edition and translation of Sallustius' Concerning the Gods and the Universe (Cambridge: Cambridge University Press, 1926), but I'm confident that the quotation in question can be found there.

Thomas Carlyle wrote to FitzGerald (January 19, 1846), "[Y]ou are, in matters small and great, a friend to light and correctness, and an enemy to darkness and error." The Terhunes' confusion of "your man Sallust" is a small matter, of course, but nevertheless one worth correcting.